The present invention relates to a screen printing apparatus and more particularly to improvements in a screen printer for simultaneously performing multiple printing operations in a continuous manner.
Screen printing is a relatively well-known process wherein ink is pressed through small holes in a screen to print designs on a substrate. Typically, ink is spread on a screen by a flood bar, and forced through the screen in small quantities by a squeegee. Modern improvements include mounting the flood bar and squeegee on a translatable carriage which shifts the flood bar and squeegee between print and non-print positions, and then sequentially translates across the screen after each shift. In the non-print sequence, the flood bar is lowered and is carried across the screen to spread the ink. In the print sequence, the squeegee is lowered and moved across the screen to print the ink.
Automatic screen printing presses are presently available for simultaneously printing at multiple stations such as for printing multiple colors. The presses move a plurality of substrate carrying platens stepwise around a main frame in a circle or oval path. Traditionally the platens move in a horizontal plane and sequentially under a plurality of printing heads, the heads defining individual stations. As the platens are stopped under each printing head, a screen and printing head first move vertically down onto individual platens to print and then up to a raised position via four corner cylinders, the screen and head poised for the next cycle. Alternatively in a separate press design, the printing heads pivot as they are raised up and down in a flat bed press, most typically all printing heads moving simultaneously. Examples of such machines are illustrated in: U.S. Patent No. Re. 29,160 issued Mar. 29, 1977 to Jaffa, and entitled: SCREEN PRINTING MACHINE WITH OVAL RAIL FOR INDEXING PALLETS and U.S. Pat. No. 4,407,195 issued Oct. 4, 1983 to Jaffa, and entitled SCREEN PRINTING MACHINE.
A revolutionary improvement was made in screen printers wherein the printing heads were held stationary while the platens pivot. An example of such a machine is illustrated in: U.S. Pat. No. 4,920,878 issued May 1, 1990 to Harpold et al, and entitled SCREEN PRINTER WITH PLATEN LIFTING STRUCTURE the entire contents of which are incorporated by reference. In these machines, one or more platens supporting articles to be printed are driven along a track, the platens moving laterally in a circle or ovally around a main frame or base. The platens are then laterally stopped and vertically lifted to a print position under stationary printing heads.
However, this improved screen printer as well as the other style continue to present further opportunity for improvement. For one, screen printing presses utilize air and hydraulic cylinders for lifting and other mechanical movements. These cylinders and the hydraulic or pneumatic systems which actuate them are expensive. Also, if not taken care of properly, these systems are subject to a variety of problems such as dirty oil or air, a frequent problem of poor maintenance which causes press malfunctions and downtime. Further, these cylinders and actuator systems are cumbersome and can be difficult to work with and around. Also, air cylinders are subject to `bounce` at the end of cylinder strokes.
Some mechanisms for replacing cylinders have been designed for shifting the flood bar and print squeegee on the carriage such as chain and sprocket mechanisms. However, these mechanisms are often cumbersome and mechanically complex. Further, the mechanisms can be difficult to work around and add expense to the screen printer. Also, they often require continuous maintenance such as lubricating. Further, the position of cylinders are usually detected and controlled by use of limit switches. However, the limit switches are inconvenient to adjust.
The present machines utilizing pivoting platens include a platen registration system which has a pair of elongated indexing assemblies that cooperate to locate individual platens with respect to the screen and printing head at each station. The registration system is illustrated in: U.S. Pat. No. 4,920,878 issued May 1, 1990 to Harpold et al, and entitled SCREEN PRINTER WITH PLATEN LIFTING STRUCTURE. However, the elongated indexing assemblies may hang up in the raised print position if not properly adjusted and maintained. This can cause press downtime and disrupt the printing process.
Another problem is that the print screens periodically need to be cleaned or inspected, or the substrate being printed on needs to be visually inspected. In a fixed head machine, and also in machines wherein the printing head is vertically lifted or tilted, cleaning or inspecting of the screen or substrate can be difficult due to the lack of space between the screen, the platen (i.e. substrate) and the printing head. Compounding this problem is the fact that operators do not want to remove the screen because then the screen must be realigned.
In presses which include a pivoting type head, the bottom of the screen is only partially accessible when the screen is up. Further the mechanism for raising and lowering the head is cumbersome since the mechanism is ruggedly designed to be a part of the printing cycle.
Thus, further improvements are desired in simplicity, cost, reliability, ease of adjustment, and flexibility of use.